In reply to maybe_si:
I went there last year and enjoyed it. We didn't do any official tour but there was a cabin offering them at the tourist centre at the highpoint of the main road here: 37.700293N, 14.998405E we had a rental car and stayed at the Corsaro hotel round the corner. It was off season so the hotel was empty like something from the Shining and it was cheap; in high season it is probably neither. If you don't have a rental car there is a public bus from Catania that takes you to this point once a day and gets there about 10AM. There is only one ride back down so don't miss it.
The DIY route is to go up on the extortionately priced cable car which gains you about 600m to 2500m or so for about €30 (one way). You could walk up a path but it's easier to suck it up and ride. At the top of the cable car you can get on a moon bus type thing which takes you a further 400m uphill and 2km. This whole area is thronged with tourists.
We chose to walk up this as it's an interesting area with great views. There are plenty of paths or you can just make your own route up, in good weather this is easy. There are some old craters to peer into, some are warm.
At the top of the bus area is Monte Frumento Supino where there are some more craters. Also here is the boundary of the summit forbidden zone. Officially you are not allowed inside this area without a guide. Roughly speaking, it's the area over 3000m around the summit but there are some official maps online that define it. Here you could either walk up to the summit or turn around, or circumnavigate the entire mountain. Being the highest point around, the peak is often cloudy from late morning onwards, so if you are not early in the day here, you won't see much. We didn't go to the very summit as it was cloudy and windy with snow still on the ground but we went some of the way. We saw some people descending through the mist from the summit, which was clear first thing in the morning. If you pay for the guide you could go to the top via the best route, and the guide will use the cable car and moon bus for speed. My parents did this as part of a group and enjoyed it, but the guide was silent so none too educational! They said the tour was rushed as it was timed around the public bus.
It's worth pointing out that this is one of the most active volcanoes in Europe and erupted quite recently so the paths may have moved about. Although the navigation is easy in good weather because the terrain is open and you can see where you're going, off the paths the lava flows can be very difficult to walk over as it's rough, sharp and moves when you stand on it. If the guides are not going up then it's for a reason so don't! A GPS was useful as the maps for the area seemed lacking (they go out of date) and the terrain is quite featureless. A personal guide might be a good idea if there has been any recent activity as they can probably take you to that area to see it from a safe vantage point, whereas a self guided trip in those circumstances might not be wise. If not under time pressure you can legitimately walk unguided to the top of the public area at 2900m from the car park at 1900m without any problems but it is hot and dusty in the sun.
We walked all the way down via quite a long route which was well worthwhile as we were not under time pressure and found some lava caves and excellent views over the Valle Bove.
Also in the area, a hike to the Grotta Degli Archi was enjoyable, 37.72758N, 14.96677E and you can explore various old lava tubes and hornitos. Again, no guide needed. There are many lava tubes in the area, if that's your thing. We found a few to crawl through, some are longer and you might want some basic caving gear eg rope as there are short drops. There are various paths in the area, some are big cycle tracks, some just footpaths not really marked eg on openstreetmap. Some might have been erased by lava flows. Take sturdy boots if you're going crashing through the lavafields! Take crap old clothes if you go in the lava tubes as they will get trashed!
Although not in answer you your question, we really enjoyed a hike up Stromboli which you might have on your itinerary. This starts in the town (~sea level) in the evening and you get to the top (900m) at sunset where you can peer into the craters which occasionally spew lava (beneath you). This is another guided tour but it's only about €15 per head in groups of 20+. The walk is not difficult and the views are super.
